1. Field of Invention
This invention relates to fluid flow measuring devices and, more particularly to an ion beam anemometer capable of measuring, unobstrusively and without disturbance, the magnitude and direction of fluid flowing therethrough.
2. Description of Prior Art
This invention is related to and derived from the prior patents of applicant U.S. Pat. No. 3,470,741, issued Oct. 7, 1969, and U.S. Pat. No. 3,831,445, issued Aug. 27, 1974. Other relevant prior art includes U.S. Pat. No. 4,331,037, issued May 25, 1982, and a publication by the applicant and a colleague entitled "An Airspeed Vector Sensor For V/STOL Aircraft" by Enoch Durbin and Tad McGeer, which was delivered Dec. 3, 1980 at an AIAA meeting in Colorado Springs, Colo.
For some time a technique for air speed sensing, using the ion beam created by a positive corona discharge, has been under development. The technique employs ions in transit across an electrode gap which may be deflected by airflow during the transit. By measuring the deflection, one can determine the flow rate.
In modern aircraft, the motion of the vehicle is determined by the forces acting upon it. In aircraft, the predominant forces are aerodynamic and the magnitude and direction of these forces are determined by the shape of the craft, and by the speed and angle of the incident airflow. One must know the forces on the aircraft if it is to be controlled. It is therefore necessary to have available to a pilot, or to an automatic control system, specific velocity information which can be resolved with respect to the three axes of the vehicle.
It is particularly important to have such velocity vector information in the low speed maneuvering of helicopters and V/STOL aircraft where optimum performance is achieved in a narrow speed range since a 1 or 2 knot error in the rotation speed (Vr) of a heavily loaded helicopter can incur significant penalties in payload and distance required to clear obstacles. Complete velocity information is often needed for safe as well as for optimum flight.
On some aircraft of the V/STOL type, sideslip must be held within a fairly narrow band during hover and transition to avoid exceeding lateral control power. However, the requirement is not limited to low speed flight. It is also necessary, for example, in the takeoff and initial climb of large, multi-engine aircraft, in which the optimum rotation speed (Vr) and climb angle of attack are important, to have precisely defined quantities.
In prior art, most aircraft have used a pitot-static system to provide velocity information. This is primarily a pressure sensing system which can be simple and unobtrusive. So long as aerodynamic angles are small, velocities exceed 30 to 40 knots, and the structure can be in the undisturbed stream, the apparatus is relatively error-free. At large angles, the errors become significant and at low speeds the device is substantially insensitive.
A major shortcoming of such a device is that only a single component of velocity can be sensed. The addition of flow angle vanes can resolve the other orthogonal components but the sensitivity of these is also reduced at lower speeds if inertial and frictional forces are of the same order of magnitude as the aerodynamic forces.
What is needed and what is supplied by the present invention is an airspeed vector sensor capable of operating over the whole of the flight envelope without disturbing the airstream. Such a device should be insensitive to environmental variation and, preferably linear in response. It should be simple, sturdy, and sufficiently "clean" to mount onto an airframe.
In the prior patents of applicant noted above, fluid velocity meters were disclosed which measured the ion drift of an ionized fluid to determine the flow. In U.S. Pat. No. 3,470,741, several embodiments were disclosed including embodiments requiring the translation of an ion source to compensate for the ion drift resulting from fluid flow.
In alternative embodiments, an ion source was fixedly mounted with a planar collector electrode that was capable of determining the amount of drift in either one or two directions in a plane orthogonal to the direction of ion drift. In part of U.S. Pat. No. 3,831,445, an alternative structure was provided which was capable of varying the electric field intensity to compensate for changes in ion mobility.
In the recently issued patent to DuVall U.S. Pat. No. 4,331,037, and assigned to a licensee of the inventor, a fluid movement measuring device was provided in which an ion beam was provided, together with a segmented detector, to detect fluid movement in directions transverse to the direction of ion flow. The ionizing means was located in a protected chamber and a protected fluid flow path was provided.
In the AIAA paper delivered by applicant and a colleague (noted above), a combination including an axially sensitive flow sensor and a transversely sensitive flow sensor was discussed in general terms. The theoretical support for such apparatus was developed without disclosing the apparatus from which experimental results to verify the theory had been derived. Footnoted therein was the master's thesis of Steve Weatherspoon, "A Flow Vector Sensitive Ion Beam Anemometer", filed in the Princeton Department of Aerospace and Mechanical Sciences, which taught that a transverse flow device could be linear and relatively insensitive to environmental change at high current.